Two-piece golf ball

ABSTRACT

The present invention is a two-piece golf ball which includes a core and a cover having a thickness of 1.0 to 3.0 mm. The core is made from a rubber composition containing a main rubber component which mainly contains polybutadiene. The cover is made from a material mainly containing an ionomer resin in which at least one kind of silicone powder selected from a silicone rubber powder, a silicon resin powder, and a composite powder thereof is dispersedly blended. The core has a hardness corresponding to a deflection in a range of 3.3 to 6.0 mm under an applied load of 100 kg. The cover has a Durometer D hardness in a range of 55 to 70.

BACKGROUND OF THE INVENTION

The present invention relates to a two-piece solid golf ball excellentin resilience and thereby flight distance characteristic, and feel ofhitting and durability.

Golf balls have been requited to be excellent in resilience and therebyflight performance, and feel of hitting upon shots and durability, andto meet such requirements, various kinds of golf balls have beenproposed, for example, in Japanese Patent Laid-open Nos. Hei 6-198005and Hei 8-294549, and U.S. Pat. Nos. 5,971,870, 6,152,835, and6,218,453B1.

The use of silicone materials for producing golf balls has beenproposed, for example, in Japanese Patent Laid-open No. 2001-170213, andU.S. Pat. Nos. 6,159,110, 6,162,134, and 6,204,331. Such golf ballsusing silicon materials, however, have been recently required by golfplayers to be further improved in terms of resilience, feel of hitting,and durability.

SUMMARY OF THE INVENTION

In view of the foregoing, the present invention has been made, and anobject of the present invention is to provide a two-piece golf ballexcellent in resilience and thereby flight distance characteristic, andfurther feel of hitting and durability.

To achieve the above object of the present invention, according to anaspect of the present invention, there is provided a two-piece golf ballincluding: a core made from a rubber composition containing a mainrubber component which mainly contains polybutadiene; and a cover havinga thickness of 1.0 to 3.0 mm, the cover being made from a materialmainly containing an ionomer resin in which at least one kind ofsilicone powder selected from a silicone rubber powder, a silicon resinpowder, and a composite powder thereof is dispersedly blended; whereinthe core has a hardness corresponding to a deflection in a range of 3.3to 6.0 mm under an applied load of 100 kg; the cover has a Durometer Dhardness in a range of 55 to 70; and dimples of the number of 300 to 550pieces are formed in the surface of the golf ball in such a manner thata dimple total volume ratio V_(R), which is a ratio of the total ofvolumes of dimple spaces under the planes surrounded by edges of thedimples in the surface of the gold ball to the total volume of a virtualball as a result of assumption that no dimple is formed in the surfaceof the golf ball, is in a range of 0.85% or less.

An average particle size of the silicon powder is preferably in a rangeof 0.5 to 700 μm.

The content of the silicon powder is preferably in a range of 0.5 to 20parts by weight on the basis of 100 parts by weight of the ionomerresin.

The rubber composition forming the core preferably contains an organicsulfur compound.

The rubber composition forming the core preferably contains a filler foradjustment of a specific gravity, the filler mainly containing zincoxide and additionally containing barium sulfate in an amount of 10parts or less by weight on the basis of 100 parts by weight of the mainrubber component.

The cover preferably contains an inorganic filler.

The inorganic filler used for the core is preferably barium sulfate.

According to the golf ball of the present invention, since the siliconepowder is dispersedly blended in the cover, the resilience of the ballis improved, which compensates for a reduction in resilience due tohardening of the ball, to thereby improve the flight performance, andfurther the temperature dependency of the ball is improved. In addition,since the core is soft, the initial condition upon hitting can be setwith a low spin rate and a high launch angle, to increase the flightdistance of the ball, and the feel of hitting upon full-shot with adriver becomes very soft. Also, since the dimple total volume isoptimized, it is possible to prevent occurrence of a drop phenomenon ofthe trajectory of the ball due to a low spin rate, and hence to obtain agood flight distance characteristic due to optimization of thetrajectory of the ball. Further, since the core contains an organicsulfur compound, it is possible to improve the resilience of the core,and since the cover contains an inorganic filler, especially, bariumsulfate, it is possible to improve the durability of the cover againstcracking.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be more fully described below.

The two-piece golf ball of the present invention includes a core and acover.

The core is formed from a rubber composition containing a main rubbercomponent. The main rubber component mainly contains polybutadiene whichpreferably contains 40% or more, especially, 90% or more ofcis-1,4-bonds. The main rubber component may contain, in addition topolybutadiene, a diene based rubber such as polyisoprene rubber,styrene-butadiene rubber, or natural rubber. The content ofpolybutadiene in the main rubber component is preferably in a range of50% (“% by weight”, the same applying correspondingly to the following)or more, especially, 70% or more.

The rubber composition used herein contains, in addition to the mainrubber component, a crosslinking agent, preferably, in an amount of 15to 40 parts (“parts by weight”, the same applying correspondingly to thefollowing) on the basis of 100 parts of the main rubber component. Thecrosslinking agent may be selected from zinc salts, magnesium salts, andother metal salts of unsaturated fatty acids such as zinc acrylate andzinc methacrylate, esters such as triethanolpropane methacrylate, andunsaturated fatty acids such as methacrylic acids.

The rubber composition may also contain an organic peroxide such asdicumyl peroxide, preferably, in an amount of 0.1 to 3 parts on thebasis of 100 parts of the main rubber component. To improve theresilience of the core, the rubber composition may further contain avulcanizing agent such as an organic sulfur compound, for example, zincsalt of pentachlorothiophenol or diphenyldisulfide in an amount of 0.01to 5 parts, especially, 0.2 to 3 parts on the basis of 100 parts of themain rubber component.

The rubber composition may further contain, if needed, an antioxidantsuch as 2,2-methylene bis(4-methyl-6-tert-buthylphenol), and a fillerfor adjustment of a specific gravity, such as zinc oxide, bariumsulfate, or calcium carbonate. The filler may be generally contained inthe main rubber component in an amount of 130 parts or less on the basisof 100 parts of the main rubber component. In particular, to improve theresilience of the core, the filler may be contained in the main rubbercomponent in an amount of, preferably, 50 parts or less, morepreferably, 45 parts or less, especially, 40 parts or less on the basisof 100 parts of the main rubber component. The lower limit of thecontent of the filler may be set to 1 part or more, especially, 3 partsor more, and further, 20 parts or more. In particular, to adjust aspecific gravity, the filler composed of a combination of barium sulfateand zinc oxide is often used; however, from the viewpoint of improvementof the resilience of the core, it is preferred for the filler to mainlycontain zinc oxide and additionally contain barium sulfate in a range of10 parts or less, especially, 0 part.

The core can be prepared from the above-described rubber composition,for example, by kneading the components of the rubber composition in anordinary kneader such as a Banbury mixer or a roll mill, and molding theresultant compound into a desired shape by a compression molding processor an injection molding process. In this molding process, vulcanizationcan be performed at a temperature of 130 to 180° C. for 10 to 60 min.

The deflection (corresponding to hardness) of the core under an appliedload of 100 kg may be in a range of 3.3 mm or more, preferably, 3.5 mmor more, more preferably, 3.7 mm or more, with the upper limit thereofbeing set to 6.0 mm or less, preferably, 5.8 mm or less, morepreferably, 5.5 mm or less. Too small a deflection of the core (toolarge a hardness of the core) tends to give a hard feel of hitting theball, and to increase the spin rates and thereby reduce the flightdistance of the ball. Too large a deflection of the core (too small ahardness of the core) tends to reduce the resilience of the core andalso the durability of the core against cracking. In addition, thediameter of the core is determined by the thickness of the cover.

The cover is made from a material mainly containing an ionomer resin.The kind of ionomer resin is, as will be described below, selected togive a specific hardness of the cover.

As a feature of the present invention, a silicone powder is dispersedlyblended in the cover by dispersedly blending at least one kind ofsilicone powder (which is a powder of previously hardened silicone)selected from a silicone rubber powder, a silicone resin powder, and acomposite powder thereof in the ionomer resin forming the cover.

An appropriate silicone rubber powder is exemplified by a fine powderhaving a highly polymerized three-dimensional structure, which isobtained by crosslinking linear dimethyl polysiloxane and/ormethylphenyl polysiloxane added with 0.05 mole % or more of vinyl groupsby using methyl hydrogen polysiloxane as a crosslinking agent, and apowder modified therefrom. In addition, the silicone rubber powder usedherein is preferable to have a true specific gravity of about 0.97.Examples of such a silicone rubber powder include commercially availableproducts such as KMP597, 598, 594, and 595 (spherical type) and X-52-875(amorphous type) from Shin-Etsu Chemical Co., Ltd.

An appropriate silicone resin powder is exemplified by a fine powder ofhardened polyorgano silsesquioxanes obtained by hardening siloxane bondsin a three-dimensional network expressed by a formula (RSiO_(3/2))_(n),and a powder modified therefrom. In the formula, it is recommended thatR be CH₃, C₆H₅, or a long-chain alkyl group. In addition, the siliconeresin powder used herein is preferable to have a true specific gravityof about 1.3. Examples of such a silicone resin powder includecommercially available products such as KMP590, X-52-1186, and X-52-854(spherical type), and X-52-821, X-52-830, and X-52-831 (modified type,for example, modified with vinyl groups, epoxy groups, amino groups, andthe like) produced by Shin-Etsu Chemical Co., Ltd.

An appropriate composite powder is exemplified by a powder obtained bycovering the above silicone rubber powder with the above silicone resin,and a powder modified therefrom. In addition, the silicone compositepowder used herein is preferable to have a true specific gravity of 1.0to 0.98. Examples of such a silicone composite powder includecommercially available products such as KMP600 and X-52-1139G producedby Shin-Etsu Chemical Co., Ltd.

The silicone powder used herein may be in the form of either amorphousparticles or spherical particles, although the silicone powder in theform of spherical particles is preferred.

Either of the silicone powders is recommended to have an averageparticle size in a range of, generally, 0.5 μm or more, preferably, 1 μmor more, more preferably, 3 μm or more, with the upper limit thereofbeing set to 700 μm or less, preferably, 500 μm or less, morepreferably, 100 μm or less. Too small an average particle size of thesilicone powder tends to cause a large amount of scattering of thepowder in the dispersion step, which is undesirable for production ofthe cover, whereas too large an average particle size of the siliconepowder tends to degrade scattering of the silicon powder and therebyreduce the durability of the cover against repetitive shots.

The content of the silicone powder may be in a range of 0.5 part ormore, preferably, 0.8 part or more, more preferably, 1 part or more onthe basis of 100 parts of the ionomer resin, with the upper limitthereof being set to 20 parts or less, preferably, 15 parts or less,more preferably, 8 parts or less on the basis of 100 parts of theionomer resin. Too small a content of the silicone powder fails tosufficiently achieve the effect of adding the silicone powder as thefeature of the present invention, whereas too large a content of thesilicone powder tends to cause a difficulty in uniform dispersion of thesilicone powder, and thereby reduce the resilience of the cover and thedurability of the cover.

The ionomer resin may further contain an inorganic filler. This iseffective to improve the durability of the ball against cracking. Apreferred inorganic filler is barium sulfate. The content of theinorganic filler may be in a range of 3 to 30 parts, especially, 5 to 25parts on the basis of 100 parts by the ionomer resin.

The Durometer D hardness of the cover, which is measured under JISK-7215, may be in a range of 55 or more, preferably, 56 or more, morepreferably, 58 or more, with the upper limit thereof being set to 70 orless, preferably, 67 or less, more preferably, 65 or less. Too small ahardness of the cover tends to reduce the resilience of the cover,whereas too large a hardness of the cover tends to give a hard feel ofhitting the ball, and reduce the durability of the cover againstcracking.

The hardness of the cover is the value obtained by measuring thehardness of a sheet formed from the same material as that of the coverunder JIS K-7215.

The thickness of the cover may be in a range of 1.0 mm or more,preferably, 1.2 mm or more, more preferably, 1.5 mm or more, with theupper limit thereof being set to 3.0 mm or less, preferably, 2.5 mm orless, more preferably, 2.3 mm or less. Too small a thickness of thecover tends to reduce the durability of the cover against cracking,whereas too large a thickness of the cover tends to give a hard feel ofhitting the ball.

The two-piece golf ball of the present invention can be produced inaccordance with a known method, and can be subjected to polishing,painting, and the like after formation of the cover in accordance with aknown method.

Like conventional golf balls, the two-piece golf ball of the presentinvention has, on its surface, a multiplicity of dimples. The number ofthe dimples may be in a range of 300 or more, preferably, 320 or more,more preferably, 340 or more, with the upper limit thereof being set to550 or less, preferably, 520 or less, more preferably, 500 or less. Toosmall the number of the dimples fails to ensure a sufficient totalvolume of the dimples, whereas too large the number of the dimplesreduces the size of each dimple, which is liable to be affected by thepaint to be applied on the surface of the golf ball, thereby failing tosufficiently achieve the ball characteristics.

With respect to the dimples formed in the surface of the golf ball, adimple total volume ratio V_(R), which is a ratio of the total ofvolumes of dimple spaces under the planes surrounded by edges of thedimples in the surface of the gold ball to the total volume of a virtualball as a result of assumption that no dimple is formed in the surfaceof the golf ball, may be in a range of 0.85% or less, preferably, 0.84%or less, more preferably, 0.83% or less, with the lower limit thereofbeing set to 0.60% or more, especially, 0.65% or more. Too large thesimple total volume ratio V_(R) tends to make the trajectory of the balllower, which may cause a drop phenomenon as a result of combination ofthe reduced spin rate, thereby reducing the flight distance of the ball,whereas too small the dimple total volume ratio V_(R) tends to make thetrajectory of the ball higher, which may cause an inconvenience that theflight distance of the ball be reduced and also the ball be liable to beaffected by wind.

The two-piece golf ball of the present invention may be formed inaccordance with the Rules of Golf, that is, with the diameter set to42.67 mm or more and the weight set to 45.93 g or less.

The two-piece golf ball of the present invention configured as describedabove is excellent in resilience, flight distance characteristic,durability, and feel of hitting.

EXAMPLE

The present invention will be described in more detail with reference tothe following examples and comparative examples, although not limitedthereto.

Examples and Comparative Examples

In accordance with a known golf ball production process, two-piece golfballs having dimensional and other characteristics shown in Table 3 wereeach produced by the steps of forming a core by using a material shownin Table 1, and forming a cover around the core by using a materialshown in Table 2.

Results of evaluating the characteristics of each of the two-piece golfballs thus obtained are shown in Table 3.

TABLE 1 Composition of Material for Core (parts by weight) ExampleComparative Example 1 2 3 1 2 3 4 1,4-polybutadiene 100 100 100 100 100100 100 (cis-structure) Zinc acrylate 22.5 23.0 22.5 22.5 20.8 26.4 28.9Dicumyl peroxide 1.4 1.4 1.4 1.4 1.4 1.4 1.4 Antioxidant 0.1 0.1 0.1 0.10.1 0.1 0.1 Zinc oxide 23.0 17.5 23.0 23.0 21.4 20.4 Zinc salt of 0.21.0 0.2 0.2 1.0 0.2 0.2 pentachlorothiophenol

TABLE 2 Compositions of Materials for Cover (parts by weight) ExampleComparative Example 1 2 3 1 2 3 4 Himilan 1706 48.5 47.5 45.5 Himilan1605 48.5 47.5 45.5 Himilan 1557 43.3 52 52 Himilan 1601 40 48 48 SurlynAD8542 40 Surlyn 7930 37 Nucrel AN4811 23 KMP597 3 4.2 5 9 Bariumsulphate 12.5

While not shown in Table 2, a UV absorber, an antioxidant, a dispersionauxiliary, and a coloring agent were suitably added to the materialsshown in Table 2.

The terms “Surlyn”, “Himilan”, “Nucrel”, and “KMP597” shown in Table 2are the trade names of the commercially available products sold by thefollowing makers:

Surlyn: EI DuPont de Nemours & Company

Himilan: DuPont-Mitsui Polychemicals Co., Ltd.

Nucrel: DuPont-Mitsui Polychemicals Co., Ltd.

KMP597: silicone rubber powder (spherical type, average particle size: 5μm), Shin-Etsu Chemical Co., Ltd.

TABLE 3 Example Comparative Example 1 2 3 1 2 3 4 Core Outer diameter(mm) 38.9 38.9 38.9 38.9 38.9 38.9 38.9 Weight (g) 36.0 35.1 36.0 36.036.0 36.0 36.0 Hardness (mm) 3.9 4.7 3.9 3.9 4.7 3.3 2.9 Cover Thickness(mm) 1.9 1.9 1.9 1.9 1.9 1.9 1.9 Hardness (D) 60 60 59 60 60 58 51Product Outer diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Weight(g) 45.2 45.3 45.2 45.2 45.2 45.2 45.2 Dimple Number (pieces) 432 392392 432 392 392 392 V_(R) (%) 0.77 0.78 0.78 0.93 0.78 0.91 0.78 FlightBackspin (rpm) 2540 2420 2550 2550 2430 2640 2900 Launch angle (degree)12.7 12.8 12.7 12.7 12.8 12.6 12.4 Flight distance (m) 205.0 204.0 205.0198.0 201.0 199.0 201.0 Initial velocity (m/s) 58.3 58.1 58.3 57.9 57.658.2 58.3 at 23° C. Initial velocity (m/s) 57.7 57.5 57.7 56.8 56.6 57.657.2 at 0° C. Feel of Driver ◯ ⊚ ◯ ◯ ⊚ X X Hitting Durability ◯ ◯ ◯ ◯ X⊚ ⊚

The hardness and thickness of each of the core, and cover, and ballcharacteristics shown in Table 3 were determined as follows:

[Hardness of Core]

The hardness of the core was determined by measuring a deflection of thecore under an applied load of 100 kg.

[Outer Diameter of Core]

The outer diameter of the core was determined by measuring outerdiameters of 5 points on the surface of the core and averaging themeasured values.

[Outer Diameter of Ball Product]

The outer diameter of the ball product was determined by measuring theouter diameters of five points on land portions (with no dimples) of theball product and averaging the measured values.

[Hardness of Cover]

The hardness of the cover was determined by preparing a sheet made fromthe same material as that of the cover and measuring the hardness of thesheet by the Durometer D hardness meter.

[Thickness of Cover]

The thickness of the cover was determined by an expression of (outerdiameter of ball product−outer diameter of core)÷2.

[Flight Performance]

The flight performance was evaluated from results of a test performedusing a hitting machine (club: driver, head speed: 40 m/s). In thistest, the spin rate, initial speed at each of 23° C. and 0° C., launchangle were measured by using a high-speed camera.

[Feel of Hitting]

The feel of hitting the golf ball was evaluated from results of a testperformed by hitting the ball with a driver by each of five top amateurplayers.

⊚: very soft

◯: soft

X: hard

[Durability]

The durability of the golf ball against cracking was evaluated fromresults of a test performed by repeatedly hitting the ball at a headspeed of 40 m/s. In this test, five balls were hit for each kind of thegolf balls prepared in Examples and Comparative Examples.

The evaluation was made in comparison with the durability of a generalgolf ball (commercially available under the trade name of ALTUS NEWINGfrom Bridgestone Sports Co., Ltd.).

⊚: superior to ALTUS NEWING

◯: equivalent to ALTUS NEWING

X: inferior to ALTUS NEWING

From the results shown in Table 3, the following becomes apparent.

Comparative Example 1

Since the hardness of each layer of the golf ball in Comparative Example1 is the same as that of each layer of the golf ball in Example 1, thefeel of hitting and the durability against cracking of the golf ball inComparative Example 1 are comparable to those of the golf ball inExample 1. On the other hand, in the golf ball in Comparative Example 1,when compared with the golf ball in Example 1, the resilience of thecover and accordingly the resilience of the ball is poor, to reduce theflight distance of the golf ball, and further the temperature dependencyof the golf ball is poor. Further, since the dimples are not optimized,a sufficient lift of the ball cannot be obtained, to cause a dropphenomenon, thereby reducing the flight distance of the ball.

Comparative Example 2

Since the hardness of each layer of the golf ball in Comparative Example2 is the same as that of each layer of the golf ball in Example 2, thefeel of hitting in Comparative Example 2 is comparable to that of thegolf ball in Example 2. On the other hand, in the golf ball inComparative Example 2, when compared with the golf ball in Example 2,the resilience of the cover and accordingly the resilience of the ballis poor, to reduce the flight distance of the golf ball, the temperaturedependency of the golf ball is poor, and the durability is poor (thegolf ball in Example 2 is intended to improve the durability by addingthe filler having a high specific gravity (barium sulfate) in thecover).

Comparative Example 3

The golf ball in Comparative Example 3 has a ball configuration similarto that of the golf ball disclosed in Japanese Patent Laid-open No.2001-170213. Since the core of the ball is hard, the feel of hitting theball becomes hard, and since the total dimple volume is not optimized, asufficient lift of the ball cannot be obtained, to cause a dropphenomenon, thereby reducing the flight distance of the ball.

Comparative Example 4

The golf ball in Comparative Example 4 is a general spin type two-piecegolf ball, and is inferior to the golf ball in each of Examples 1, 2 and3 in terms of flight distance due to a large spin rate, and further thegolf ball in Comparative Example 4 gives a hard feel when hit with adriver.

On the contrary, the golf ball in each of Examples 1, 2 and 3 exhibitsgood flight performance, feel of hitting, and durability.

While the preferred embodiment and examples of the present inventionhave been described using specific terms, such description is forillustrative purposes only, and it is to be understood that changes andmodifications may be made without departing from the spirit and scope ofthe following claims.

What is claimed is:
 1. A two-piece golf ball comprising: a core madefrom a rubber composition containing a main rubber component whichmainly contains polybutadiene; and a cover having a thickness of 1.0 to3.0 mm, said cover being made from a material mainly containing anionomer resin in which at least one kind of silicone powder selectedfrom a silicone rubber powder, a silicon resin powder, and a compositepowder thereof is dispersedly blended; wherein said core has a hardnesscorresponding to a deflection in a range of 3.3 to 6.0 mm under anapplied load of 100 kg; said cover has a Durometer D hardness in a rangeof 55 to 70; and dimples of the number of 300 to 550 pieces are formedin the surface of said golf ball in such a manner that a dimple totalvolume ratio V_(R), which is a ratio of a total of volumes of dimplespaces under planes surrounded by edges of the dimples in the surface ofsaid gold ball to a total volume of a virtual ball as a result ofassumption that no dimple is formed in the surface of said golf ball, isin a range of 0.85% or less.
 2. A two-peace golf ball according to claim1, wherein an average particle size of said silicon powder is in a rangeof 0.5 to 700 μm.
 3. A two-piece golf ball according to claim 1, whereinthe content of said silicon powder is in a range of 0.5 to 20 parts byweight on the basis of 100 parts by weight of said ionomer resin.
 4. Atwo-piece golf ball according to claim 1, wherein said rubbercomposition forming said core contains an organic sulfur compound.
 5. Atwo-piece golf ball according to claim 1, wherein said rubbercomposition forming said core contains a filler for adjustment of aspecific gravity, said filler mainly containing zinc oxide andadditionally containing barium sulfate in an amount of 10 parts or lessby weight on the basis of 100 parts by weight of said main rubbercomponent.
 6. A two-piece golf ball according to claim 1, wherein saidcover contains an inorganic filler.
 7. A two-piece golf ball accordingto claim 6, wherein said inorganic filler is barium sulfate.